As electronic memories approach limits beyond which they will no longer be able to produce the density/cost/performance improvements so famously set forth in Moore's law, a host of memory technologies are being investigated as potential replacements for conventional silicon complementary metal oxide semiconductor (CMOS) integrated circuit memories. Among the technologies being investigated are programmable resistance technologies, such as phase change memory technologies. Phase-change memory arrays are based upon memory elements that switch among two material phases, or gradations thereof, to exhibit corresponding distinct electrical characteristics. Alloys of elements of group VI of the periodic table, such as Te, S or Se, referred to as chalcogenides or chalcogenic materials, can be used advantageously in phase change memory cells. In some chalcogenide materials, the resistivity varies by two or more orders of magnitude when the material passes from the amorphous (more resistive) phase to the crystalline (more conductive) phase, and vice versa. Further, the resistivity of the chalcogenide materials generally depend on the temperature with the amorphous state generally being more temperature dependent that the crystalline state.
A chalcogenide memory device may utilize the wide range of resistance values available for the material as the basis of memory operation. Each resistance value corresponds to a distinct structural state of the chalcogenide material and one or more of the states can be selected and used to define operational memory states. Chalcogenide materials exhibit a crystalline state, or phase, as well as an amorphous state, or phase. Different structural states of a chalcogenide material differ with respect to the relative proportions of crystalline and amorphous phase in a given volume or region of chalcogenide material. A chalcogenide memory device's range of resistance values is generally bounded by a set state and a reset state of the chalcogenide material. By convention, the set state is a low resistance structural state whose electrical properties are primarily controlled by the crystalline portion of the chalcogenide material and the reset state is a high resistance structural state whose electrical properties are primarily controlled by the amorphous portion of the chalcogenide material.
Phase change may be induced by increasing the temperature locally. Below 150° C., both of the phases are stable. Above 200° C., there is a rapid nucleation of the crystallites and, if the material is kept at the crystallization temperature for a sufficiently long time, it undergoes a phase change and becomes crystalline. To bring the chalcogenide back to the amorphous state it is necessary to raise the temperature above the melting temperature (approximately 600° C. for GST 225, for example) and then cool it off rapidly, i.e. quench. From the electrical standpoint, it is possible to reach the crystallization and melting temperatures by causing a current to flow through a crystalline resistive element that heats the chalcogenic material by the Joule effect.
Each memory state of a chalcogenide memory material corresponds to a distinct range of resistance values and each memory resistance value range signifies unique informational content. Operationally, the chalcogenide material can be programmed into a particular memory state by providing an electric current pulse of an appropriate amplitude and duration to transform the chalcogenide material into the structural state having the desired resistance. By controlling the amount of energy provided to the chalcogenide material, it is possible to control the relative proportions of crystalline and amorphous phase regions within a volume of the material and to thereby control the structural (and corresponding memory) state of the chalcogenide material to store information.
Each memory state can be programmed by providing the current pulse characteristics of the state and each state can be identified, or “read”, in a non-destructive fashion by measuring the resistance of the material. The variable resistance memory functionality of chalcogenide materials is currently being exploited in the OUM (Ovonic Universal (or Unified) Memory) devices that are beginning to appear on the market. Basic principles and operation of OUM type devices are presented, for example, in U.S. Pat. Nos. 6,859,390; 6,774,387; 6,687,153; and 6,314,014; the disclosures of which are incorporated by reference herein, as well as in several journal articles including, “Low Field Amorphous State Resistance and Threshold Voltage Drift in Chalcogenide Materials,” published in EE transactions on Electron Devices, vol. 51, p. 714-719 (2004) by Pirovano et al.; and “Morphing Memory,” published in Science News, vol. 167, p. 363-364 (2005) by Weiss.
The behavior (including switching, memory, and accumulation) and chemical compositions of chalcogenide materials have been described, for example, in the following U.S. Pat. Nos. 6,671,710; 6,714,954; 6,087,674; 5,166,758; 5,296,716; 5,536,947; 5,596,522; 5,825,046; 5,687,112; 5,912,839; and 3,530,441, the disclosures of which are hereby incorporated by reference. These references present proposed mechanisms that govern the behavior of chalcogenide materials. The references also describe the structural transformations from the crystalline state to the amorphous state (and vice versa) via a series of partially crystalline states in which the relative proportions of crystalline and amorphous regions vary during the operation of electrical and optical programming of chalcogenide materials.
A wide range of chalcogenide compositions has been investigated in an effort to optimize the performance characteristics of chalcogenic devices. Chalcogenide materials generally include a chalcogen element and one or more chemical or structural modifying elements. The chalcogen element (e.g. Te, Se, S) is selected from column VI of the periodic table and the modifying elements may be selected, for example, from column III (e.g. Ga, Al, In), column IV (e.g. Si, Ge, Sn), or column V (e.g. P, As, Sb) of the periodic table. The role of modifying elements includes providing points of branching or cross-linking between chains comprising the chalcogen element. Column IV modifiers can function as tetracoordinate modifiers that include two coordinate positions within a chalcogenide chain and two coordinate positions that permit branching or crosslinking away from the chalcogenide chain. Column III and V modifiers can function as tricoordinate modifiers that include two coordinate positions within a chalcogenide chain and one coordinate position that permits branching or crosslinking away from the chalcogenide chain. Embodiments in accordance with the principles of the present invention may include binary, ternary, quaternary, and higher order chalcogenide alloys. Examples of chalcogenide materials are described in U.S. Pat. Nos. 5,166,758, 5,296,716, 5,414,271, 5,359,205, 5,341,328, 5,536,947, 5,534,712, 5,687,112, and 5,825,046 the disclosures of which are all incorporated by reference herein. Chalcogenide materials may be deposited with a reactive sputtering process with gasses such as N2 or O2: forming a chalcogenide nitride, or oxide, for example and chalcogenide may be modified by an ion implantation or other process. Materials may also be deposited using chemical vapor deposition (CVD) processes, for example.
Significant research and development efforts have been expended in an attempt to bring programmable resistance memories to full-scale production including multi-level cell embodiments. One impediment to wide-spread adoption of the multi-level cell technology is the tendency the devices' resistance to increase over time. That is, the resistance of a programmable resistance memory (a phase change memory in particular) programmed to a nominal resistance level may increase over time. That resistance increase may, over time, render the programmed state of the device ambiguous. In multi-level cell embodiments, where an individual memory element is programmed to one of three or more resistance values, this phenomenon, generally referred to as, resistance drift, or, simply, drift, is particularly troublesome and a number of drift-compensation techniques have been proposed. Resistance drift can generally be described in terms of a power law function where the resistance changes as a function of time to a certain power d, and a shorthand measure of the resistance drift's magnitude is the power law factor, d. R=R0(t/t0)d where R0 is the resistance at a time t0 after programming and d is the drift power coefficient. The coefficient d will be referred to herein as the resistance drift coefficient or, simply, as the drift coefficient.
Although already efficient and cost effective; manufacturing and operational processes, and device structures, that further reduce the cost of programmable resistance memories, and, in particular, that address the problem of resistance drift would be highly desirable.